A multi-blade centrifugal fan in which an impeller having a plurality of blades is installed in a scroll casing that has its starting point at a tongue portion is widely applied to blower fans of refrigerating devices, air conditioning devices, or ventilation devices, etc. (hereinafter, simply referred to as air conditioners). In such a multi-blade centrifugal fan, air taken in, in an axial direction, from an inlet provided in a top surface of the scroll casing with the rotation of the impeller passes through between the plurality of blades of the impeller, is forcedly supplied from an inner circumferential side to an outer circumferential side, thereby changing its direction to a centrifugal direction (radial direction), is made to flow out to an air channel in the scroll casing from the impeller, and is subsequently sent in a circumferential direction along an inner circumferential surface thereof to be blown out to the exterior via an outlet.
With such a multi-blade centrifugal fan, it is known that flow reversal toward the impeller occurs near the tongue portion of the scroll casing and that an abnormal noise (irritating noise) is generated by interference between the impeller and the flow in the reverse flow region, vibrations due to turbulence in the flow and vortices in the reverse flow region, as well as interference between the scroll casing and turbulence in the main flow or the vortices, and so on. As a measure against this, Patent Literature 1 proposes a scroll casing whose air-channel bottom surface is inclined downward radially outward from a position at a lower portion of an outer circumferential end of the impeller, thus suppressing the occurrence of vortices.
In addition, in the disclosure in Patent Literature 2, a twisted surface where the angle of an inclined surface thereof increases from a spiral-end portion of the scroll casing toward an outlet region is formed, and a secondary flow flows along the twisted surface so as to be expanded radially inward, thereby preventing interference with the main flow. Furthermore, in the disclosures in Patent Literatures 3, 4, etc., a rib or a secondary-flow suppression vane is provided along the airflow direction at a bottom surface in a region closer to an exit of an air channel of the scroll casing, and a secondary flow toward the impeller is suppressed thereby to reduce noise.